Wind turbine control having a lidar wind speed measurement apparatus

ABSTRACT

A wind turbine is provided having a lidar wind speed measurement apparatus for achieving wind control. The lidar apparatus is arranged to scan the area in front of the wind turbine so as to generate a measurement of the wind speed across the wind field. The lidar apparatus may be located in the hub of the wind turbine and the look direction inclined away from the rotational axis so that rotation of the hub ensures scanning. Preferably the lidar apparatus has a plurality of look directions so as to increase the scanning rate. This may be achieved by having a number of dedicated lidar systems and/or by using multiplexed lidars. Measurement of the wind field allows improved control of the wind turbine giving efficiency and reduced wear benefits.

This application is the U.S. national phase of international applicationPCT/GB2004/000841 filed 26 Feb. 2004 which designated the U.S. andclaims benefit of GB 0304603.4 filed 28 Feb. 2003, the entire contentsof each of which are hereby incorporated by reference.

This invention relates to a control system for a wind turbine and towind turbines having lidar systems to provide pitch blade control.

Wind turbines are growing in popularity as a means of generating energydue to their renewable nature and lack of pollution. The wind turbinesgenerally have a rotor with two or three blades connected to agenerator.

The efficiency with which a wind turbine can extract power from the windwill depend on various factors. It is known that maintaining a constanttip speed to wind speed ratio can improve the performance of some windturbines. This however requires knowledge of the wind speed. U.S. Pat.No. 4,331,881 discloses a field control system for wind drivengenerators in which the wind speed is determined by an anemometer, suchas a cup anemometer, and the field current of the generator controlledso as to load the turbine to maintain a constant tip speed to wind speedratio.

Cup anemometers and the like however suffer from the disadvantage thatwhen mounted on the turbine they only give an indication of the windspeed at the turbine. Altering the turbine characteristics can take afinite amount of time and therefore ideally the wind speed a shortdistance in front the turbine is required so that the correct settingcan be implemented. It is known to place anemometers on masts ahead ofthe wind turbine but, given that the turbine rotates to face the wind,the mast may not always be correctly positioned in front of the turbine.

Laser radar (Lidar) systems have been known for measuring wind speed anddirection for many years. Typically, they have employed CO₂ lasersystems and have been successfully employed on a wide range ofapplications. Typically, the lidar operates by scattering radiation fromnatural aerosols (dust, pollen, water droplets etc.) and by measuringthe Doppler shift between the outgoing and returning radiation. In orderto measure wind speed and direction it is usual to scan the lidar,typically using a conical scan, so that the wind vector may beintersected at a range of angles, enabling the true (3D) velocity vectorto be deduced. Other scanning patterns could be used to determine thetrue vector, provided the lidar pointing direction is always known witha high degree of accuracy. Such lidars have been used to measure windshear, turbulence and wake vortices for many years in both military andcivil applications.

Laser Doppler Velocimetry Applied to the Measurement of Local and GlobalWind, J. M Vaughan and P. A. Forrester, Wind Engineering, Vol. 13 No. 11989 describes how a lidar system can be used to take wind measurementsahead of a wind turbine to allow control of blade pitch for mostefficient operation.

WO98/42980 teaches that a laser anemometer can be mounted on a windturbine so as to follow motion of the nacelle. In other words the lidarsystem can be arranged, for instance by mounting on the nacelle, toalways look at the same position relative to the nacelle. In this waythe lidar system always gives an indication of the wind speed a certaindistance upwind. This allows a controller to set an appropriate bladepitch for the detected wind speed to maintain a constant tip speed towind speed ratio.

The wind speed data collected by the apparatus described in WO98/42980is limited however and only basic control of the wind turbine ispermitted.

It is therefore an object of the invention to provide an improved windturbine control system.

Thus according to the present invention there is provided a wind turbinehaving a lidar means for determining wind speed wherein the lidar meansis mounted in the hub of the turbine and has at least one look directioninclined to the axis of rotation thereof such that as the hub rotatesthe lidar means scans the area in front of the turbine.

The wind speed in front of a turbine is unlikely to be a uniform windspeed field and variations in the wind speed across the area swept outby the blades can affect the way that the turbine operates. By scanningthe lidar means across the area in front of the turbine, i.e. the areain front of the direction that the nacelle is currently pointing, thewind velocity field can be determined which can aid control of the windturbine. Some examples of improved control schemes will be describedlater.

Mounting the lidar means in the hub allows the rotation of the hub, asit is driven by the wind, to provide the motive scanning means. This notonly removes the need for complex scanning mechanisms but hub mountingalso means that the lidar means is not obstructed at any point by theblades of the wind turbine, unlike nacelle mounted systems.

A lidar means with only one look direction inclined off axis will takethe time taken for one revolution of the hub to complete a scan. Withlarge wind turbines the revolution rate can be relatively slow. In orderto ensure that wind speed data is acquired from all directions withsufficient frequency the lidar means may have a plurality of lookdirections. For instance three lidar look directions, all at the sameangle or otherwise arranged to scan the same area, could be providedequally spaced around the hub. One revolution of the hub would thenresult in the same area in front of the turbine being scanned threetimes. More than three look directions could be provided as required,for instance four or six directions.

Additionally or alternatively at least two look directions could bearranged so as to scan different areas when the hub is rotated. Forinstance two look directions inclined at different angles to the hubwould scan different areas when the hub is rotated. In this way moredetailed information about the wind field in front of the turbine may bebuilt up. For instance three look directions could be provided at oneangle to the axis of rotation and another three look directions providedat a lower angle, each group of three look directions being spacedequidistantly around the hub. This would result in two conical scanpatterns being traced. A scan corresponding to the outer cone would betraced by the three look directions at the higher angle and an innercone traced by the three look directions at the lower angle. Both coneswould be swept three times every revolution. One look direction couldalso be arranged to lie along, or be parallel to, the axis of rotation.

The lidar means could comprise a number of separate lidars having singlelook direction. Each lidar would have its own laser, transmit andreceive optics and detector. This is a simple means of achievingmultiple look directions without loss of transmit power. However thecost of having several look directions can be reduced by utilisingmultiplexed lidar apparatus to provide at least some of the lookdirections. The multiplexed lidar apparatus has one laser source but twoor more sets of transmit/receive optics to allow beams to be sent indifferent look directions. The multiplexed lidar apparatus may be of theswitched kind, where a single beam is cyclically switched into differenttransmit/receive optics, or of the beam splitting kind where the laserbeam is split into different transmit beams. The switched kind ofmultiplexed lidar would need to be operated at three times the frequencyof three separate single beam lidars to provide the same level of data.The beam splitting type can operate at the same frequency but doesrequire a separate detector for each channel and obviously output beampower is reduced.

If required the lidar means could be provided with a scanning means. Thescanning means could scan, in use, at least one look direction relativeto the hub. A relatively simple scanning means, such as a rotating prismor mirror could be used which, together with the hub rotation, couldgive complex scan patterns. However a scanning means would generallyrequire a moving optical system located within the hub which addscomplexity.

Preferably at least one look direction is inclined in the range of5°-20° of the axis of rotation and more preferably within the range of10°-20° of the axis of rotation. An off axis angle in this range, sayinclined substantially 15° to the axis of rotation, gives a good scan ofthe wind field in front of the turbine. This allows the wind speed at asufficiently wide field of view to be determined. Wind changes may notnecessarily come from directly in front of the wind turbine andmonitoring the wind field across a wide field of view can detect windchanges coming from off axis.

The wind field measurement from the lidar means is preferably input to acontrol means to control the wind turbine. One useful control is tocontrol the pitch of the rotor blades.

One use for the present invention is in gust control. Sudden changes inwind speed at the turbine can exert undue loading on one or more bladesand lead to increased stress. This in turn can cause fatigue, whichresults in a shorter lifetime or more frequent servicing for theturbine. Using a lidar, gusts can be detected well before the windchange reaches the turbine tower. Given enough notice (typically a fewseconds) the blades could be feathered (using the pitch control commonon larger turbines) thereby reducing the excess loading which a gustwould cause. In this way wear could be reduced and operational lifeextended. In extreme cases such a mechanism could prevent damage fromoccurring.

Gusts, by their very nature, may not come from directly in front of theturbine. The gust detection system described herein therefore enablesoff-axis wind changes to be detected as well.

In a preferred embodiment the control means is adapted to independentlyalter the pitch of each blade as it rotates. Because wind speed normallyincreases with height it is quite usual for the wind pressure on theuppermost blade to be much higher than that on the lower blades. Thiscan lead to an imbalance in the load on the transmission train. However,variations in wind speed over the disc could be balanced out bydynamically varying the individual blades during each rotation, i.e.load balancing. This would improve the balance on the drive train,reduce wear and improve lifetime. Ideally, one lidar beam per bladewould measure wind speed in front of the rotor at a point immediately infront of the position a given blade will reach by the time the windreaches that position.

Under different control regimes it might be possible to extract moreenergy from a varying wind by dynamically feathering the blades to theoptimum angle. This type of control could, for instance, be used inconjunction with load balancing. When winds are relatively weak (andloads small) it may be preferential to vary the pitch of each blade asit sweeps around so as to extract the maximum amount of energy from thewind. Of course, this would be contrary to the principles of loadbalancing but at moderate wind speeds this may not be so important.However, as the wind speed increases, especially above that required formaximum output power, then the control regime could switch to loadbalancing instead. In this way the same lidar sensor could be used tomaximise energy production in most conditions whilst affording greaterprotection in high and extreme winds.

Whilst the preferred embodiment of the invention uses a hub mountedlidar means it would be possible to mount the lidar means elsewhere. Forinstance a lidar system could be mounted on the nacelle and the lookdirection directed towards a mirror located in the hub on the axis ofrotation. Rotation of the hub would then scan the mirror and provide offaxis scanning. This could be achieved by directing a lidar beam througha hollow main axle. Some turbines do have offset gearboxes and do havehollow main axles. Alternatively a scanning optical system could bemounted in the nacelle. Multiple lidar look directions may be employedaround the nacelle and scanned in different directions to scan the windfield in front of the nacelle. Therefore according to a second aspect ofthe invention there is provided a wind turbine having a lidar meansarranged to scan the area in front of the turbine in a plurality of lookdirections. Preferably the lidar means is a multiplexed lidar apparatus,i.e. a lidar apparatus having a single laser coupled to two or more setsof transmit/receive optics.

The present invention therefore looks upwind and scans the wind fieldahead of the wind turbine so as to allow control thereof. There is alsobenefit however in mounting a lidar system looking downwind of the windturbine, i.e. looking backwards. By mounting a rearward facing lidarinformation about the wind field after it has passed the turbine, i.e.the turbine wake, can be collected. This information would includeinformation about the turbulence caused by the wind turbine. Knowledgeof the flow in the wake of the turbine can aid modelling of the turbineperformance which could be used in a control system for optimisingperformance. Preferably the rearward looking lidar may be a scanninglidar to scan the region of interest.

A further refinement would be to use the scanning lidar means toquantify the energy input to the wind turbine. This could provide a moreaccurate wind field measurement than current mast-mounted anemometertechniques and it would be independent of turbine azimuth orientation.Also, it would provide a more exact measure of input wind energy than asingle staring beam as described in WO98/42980. Such information couldbe used to provide an accurate determination of turbine Power Curve —animportant measure of turbine performance. The Power Curve could becontinuously monitored by an embedded lidar system designed for loadbalancing or gust protection as described above. Alternatively, thePower Curve could be measured using a separate lidar means designed tobe temporarily mounted on a turbine specifically for this purpose andthen moved from turbine to turbine making measurements as required.

The invention will now be described by way of example only with respectto the following drawings of which;

FIG. 1 shows a schematic of a lidar mounted off axis in the hub of awind turbine,

FIG. 2 shows the front view of the hub and blades of a wind turbineprovided with three lidar look directions,

FIG. 3 shows a schematic of a lidar apparatus mounted in the hub of awind turbine, and

FIG. 4 shows schematic of a multiplexed lidar apparatus having aplurality of look directions.

FIG. 1 illustrates a wind turbine having a lidar system mounted in thehub and having a look direction inclined to the axis of rotationthereof. The turbine consists of a tower 2 bearing a nacelle 4. Thenacelle 4 is connected to a rotating hub 6 which bears the blades 8.Three blades are common in modern wind turbines.

The nacelle 4 is at least partly rotatable in a plane orthogonal to thetower 2 so that the turbine always faces into the wind for maximum powerextraction. The pitch of the blades 8 is controllable by an actuatorlocated in the hub so as to vary the force experienced by the blades.Typically the pitch of the blades is varied to maximise efficient powerextraction but in strong winds the blades may be feathered to protectthe turbine.

WO98/42980 describes how a laser anemometer may be mounted on thenacelle 4 so as to determine the wind speed a certain distance in frontof the turbine which gives advance warning of the wind conditions andallows for appropriate action to be taken.

The wind field in front of a turbine is not usually uniform however.This can lead to different conditions applying across the disc swept outby the blades 8, especially with the large turbines currently beingbuilt. For instance it is usual that the wind speed increases as oneprogresses upwards from the ground. Therefore the load of the blade(s)at the top of the turbine can be greater than that on the lower blade orblades. This can create a load imbalance. In strong winds this loadimbalance can be significant and can lead to excessive wear on theturbine transmission. In less strong winds the load imbalance may not begreat but the pitch determined for the wind speed at the middle of thedisc swept by the blades may not be the most efficient.

Also gusts may not come from directly in front of the turbine andtherefore gusts from off axis can arrive at the turbine and causedamage.

In one embodiment of the present invention therefore a lidar is locatedin the hub 6 and inclined with its look direction off axis. This isshown in more detail in FIG. 3.

The lidar head and electronics 10, i.e. laser source and detector, arelocated in a sealed unit on the axis of rotation to minimise vibration.Connections, 12 and 14 respectively, to a power source and control unitin the nacelle (not shown) are via slip rings between the hub and thenacelle. Alternatively the output from the lidar could be communicatedby a fibre optic link or by wireless communication. The control unit maybe located in the hub with the lidar unit, although it may still bewished to communicate the wind speed data outside, for instance to acentral control unit for monitoring purposes.

A fibre optic 16 links the lidar head to transmit and receiveoptics—telescope 18. Telescope 18 is located in a tube 20 and positionedat an angle to the hub axis 22. The tube reduces the amount of dirt andprecipitation that reaches the front optical window 24 through theopening 26 in the hub. Drain holes 28 in the tube 20 keep the tube dry.In practice this might not be sufficient and to maintain a clean frontsurface to the optic it might be better to blow clean dry air outthrough the tube. A simple passive compressor using the incident windpassed through a filter and dryer would probably suffice. Otherwise anactive, fan could be employed. It may also be prudent, particularly incoastal regions where salt deposits occur, to incorporate a simplewash-wipe capability such as are frequently used to clean the headlampsof many motor cars.

Rotation of the hub due to the wind will therefore scan the lidar aroundthe area in front of the turbine. Referring back to FIG. 1 it can beseen that a conical scan pattern 30 is achieved by a single lidarinclined at an angle to the axis.

The choice of offset angle (to the hub axis) will depend on the extentto which wind gusts are expected to arrive at non-normal angles. It willalso depend on the choice of sampling position in front of the turbineblades. These parameters will vary from one turbine design to anotherand may also vary with the exact site of the turbine. A simple focusmechanism in the telescope will allow the lidar probe position to beeasily adjusted, either during installation or dynamically duringturbine operation. An angle of approximately ±15° to the axis wouldprovide good coverage.

The skilled person would understand that any lidar system capable ofdetermining wind speed could be used. However a particularly usefullidar system is described in WO01/35117, the contents of which isincorporated herein by reference thereto, especially the embodimentdescribed on page 5, line 25 to page 7, line 16.

With large wind turbines the rate of revolution of the hub can reachrates as low as 10 revolutions per minute and larger turbines may haveeven slower rotation rates. Therefore a single lidar with a single lookdirection would take approximately 6 seconds to complete a scan. Thismay well be too slow to provide useful wind field data for control ofthe turbine.

In another embodiment of the present invention therefore it is proposedto provide a lidar system with multiple look directions. FIG. 2illustrates a front view of a hub provided with three lidar lookdirections. In this example the three look directions are all arrangedso that the area scanned by each look direction is the same and the lookdirections are spaced equidistantly. Referring back to FIG. 1 such anarrangement would scan the conical scan area three times each revolutionor, in other words, each part of the scan would be repeated every 2seconds, which should give sufficient information.

Of course more look directions could be used if desired, six lookdirections repeating the same scan would-provide an update every second.The look directions could also be arranged to scan different parts ofthe wind field to provide more complete information. One look directioncould even be arranged on, or parallel to, the hub axis.

Achieving a number of look directions could easily be achieved byproviding a number of lidars as described above each having a telescopearrangement as described with reference to FIG. 3 pointing in adifferent direction.

However in some circumstances it may be desired to use a multiplexedlidar apparatus. A multiplexed lidar apparatus is one having a singlelaser source connected to two or more sets of transmit/receive optics.For instance the lidar head 10 in FIG. 3 could be linked to three, say,differently arranged telescopes.

FIG. 4 shows a schematic of a suitable multiplexed lidar apparatus.

A laser source 11 emits a laser beam that is coupled into an opticalfibre cable 42. A beam splitter 44 is provided and directs a smallfraction of the laser power as a local oscillator signal to opticalfibre cable 46, and the remaining optical power is directed in tooptical fibre cable 48. A person skilled in the art would recognise thatthe optical power of the local oscillator signal would advantageously beadjusted to give optimised shot noise domination in the detector.

A three way beam splitter 50 equally divides the laser power incidentfrom optical fibre cable 48 between the optical fibre cables 52 a, 52 band 52 c, which in turn are coupled to transceivers 54 a, 54 b and 54 c.Each of the transceivers 54 transmit the laser radiation, and alsooutput any received radiation (i.e. radiation reflected back to it froman object) to their respective optical fibre cables 56.

Optical mixers 58 coherently mix the received radiation of each of theoptical fibre cables 56 with the local oscillator signal provided by thebeam splitter 44. The resultant coherently mixed signals are outputalong optical fibre cables 59 to each of the respective detection means27. A personal computer (or dedicated processor) 60 processes the dataprovided by each of the detection means 27 generating range or speeddata as required. The device thus provides three simultaneousmeasurements of range and/or speed for the three transceivers; howeverthis is at the cost of each transceiver requiring its own detectionmeans 27.

Alternatively instead of the beamsplitter 50 an optical switch could beprovided to receive radiation from optical fibre cable 48, and directthat radiation to any one of the transceivers 54 a, 54 b and 54 c viathe respective optical fibre cables 52 a, 52 b or 52 c. Each transceiver54 also couples any radiation received (i.e. any returned radiation)back into the relevant optical fibre cables 52, and the optical switchwould then directs this radiation from the selected optical fibre cable52 to a fibre optical cable for mixing with the LO signal andtransmission to a single detector. Range and speed information, asrequired, can then be calculated by the personal computer 60 for theparticular selected transceiver.

The optical switch would thus has the effect of routing optical power toone transceiver, and routing the return signal received by thattransceiver to the detection means 27 thereby providing range or speedinformation. By switching the optical switch, the transceivers can besequentially activated, allowing quasi-simultaneous measurements to beperformed.

The optical switch could be any device that is capable of routingoptical signals without any significant loss of the coherenceinformation. Such switches are commonly used in the field oftelecommunications.

The result of the wind field measurement could then be used to provideimproved control of the wind turbine. As mentioned, gusts from off axiscan be detected and the blades of the turbine feathered to preventdamage.

Measurement of the different wind speeds across the disc swept by theblades would however allow the pitch of each individual blade to bealtered as it rotates. As mentioned, larger turbines have slowerrotational rates and it is possible to adjust the pitch of the blade asit turns.

The pitch of the blades could then be moved to control the load acrossthe blades and achieve load balancing. This would be useful in strongWinds as mentioned to prevent excessive wear of the transmission. Inless strong winds, when load balancing is not an issue and maximumefficiency is required the pitch of each blade could be altered toensure it is performing at maximum efficiency throughout the wholerevolution.

It is also important to determine whether gust fronts maintain theircoherence over the few hundred metres in front of the wind turbine. Thelidar of the present invention allows identification of the gust frontat some distance followed by scanning closer to the turbine to await itsarrival. The propagation of gusts can be monitored by correlating thewind speeds at the different ranges. The range gate settings can bevaried to examine correlation over different distances and to measurethe delay in arrival time allowing appropriate control of the turbine.The data collected by the turbine can also be downloaded for longer termanalysis leading to improvements in turbine design.

The airflow behind the rotor plane is also of interest to turbinemanufacturers and wind farm site developers. A detailed characteristicof this flow in the wake would assist modelling of turbine performanceand could be used in a control system to control turbine settings foroptimal operation. Further, in siting individual turbines within a windfarm the so called shadowing effect of a wind turbine must beappreciated, i.e. the effect on wind flow a turbine has that may affectother turbines located downwind. Indeed in existing wind farmsmeasurement of the wake from a turbine could be used in controlling thatturbine or other turbines so as to maximise the efficiency of the windfarm as a whole. Thus a lidar system may be mounted on the wind turbineso as to measure the airflow downwind of the wind turbine, i.e. arearward looking lidar may be mounted in or on the nacelle of a windturbine. Preferably the lidar is a scanning lidar to scan the region ofinterest behind the turbine for instance to probe the region likely tobe affected by shadowing. Alternatively multiple lidars, or a lidar withmultiple look directions, may be used to look at fixed points in spacerelative to the nacelle in the downwind region.

The information collected by a downwind pointing Lidar system would asmentioned be useful for analysis of the performance of wind turbineswhich could be used in designing better turbines in the future.Understanding the wake from wind turbines would also be beneficial inunderstanding the siting of wind farms and the effects they have on theenvironment as well as improving the siting of individual turbineswithin a wind farm.

1. A wind turbine having a lidar means for determining wind speedwherein the lidar means is mounted in the hub of the turbine and has atleast one look direction inclined to the axis of rotation thereof suchthat as the hub rotates the lidar means scans the area in front of theturbine, said lidar means comprising a multiplexed lidar, saidmultiplexed lidar comprising: at least one laser source; and a pluralityof sets of transmit/receive optics, each set of transmit/receive opticshaving a different look direction.
 2. A wind turbine as claimed in claim1 wherein at least one look direction is inclined at an angle within therange of 5°-20° of the axis of rotation.
 3. A wind turbine as claimed inclaim 2 wherein at least one look direction is inclined at an anglewithin the range of 10°-20° of the axis of rotation.
 4. A wind turbineas claimed in claim 1 further comprising a control means, responsive tothe output of the lidar means, for controlling the pitch of the rotorblades.
 5. A wind turbine as claimed in claim 4 wherein the controlmeans is adapted to feather the rotor blades when incoming wind gustsare detected.
 6. A wind turbine as claimed in claim 4 wherein thecontrol means is adapted to independently alter the pitch of each bladeas it rotates.
 7. A wind turbine as claimed in claim 6 wherein thecontrol means is adapted to alter the pitch of each blade to maximiseenergy extraction.
 8. A wind turbine as claimed in claim 6 wherein thecontrol means is adapted to alter the pitch of each blade to provideload balancing.
 9. A wind turbine having a lidar for determining windspeed wherein the lidar is mounted in the rotating hub of the turbine,said lidar comprising a multiplexed lidar comprising: at least one lasersource; and a plurality of sets of transmit/receive optics, each set oftransmit/receive optics having a different look direction wherein as thehub rotates the lidar scans the area in front of the turbine.
 10. A windturbine as claimed in claim 9 wherein said different look directions areaccomplished simultaneously.
 11. A wind turbine as claimed in claim 9wherein there are three different look directions.
 12. A wind turbine asclaimed in claim 9 wherein at least one look direction is inclined at anangle within the range of 5°-20° of the axis of rotation.
 13. A windturbine as claimed in claim 12 wherein at least one look direction isinclined at an angle within the range of 10°-20° of the axis ofrotation.
 14. A wind turbine as claimed in claim 9 further comprising acontrol means, responsive to an output of the lidar, for controlling thepitch of the rotor blades.
 15. A wind turbine as claimed in claim 14wherein the control means is adapted to feather the rotor blades whenincoming wind gusts are detected.
 16. A wind turbine as claimed in claim14 wherein the control means is adapted to independently alter the pitchof each blade as it rotates.
 17. A wind turbine as claimed in claim 16wherein the control means is adapted to alter the pitch of each blade tomaximise energy extraction.
 18. A wind turbine as claimed in claim 16wherein the control means is adapted to alter the pitch of each blade toprovide load balancing.
 19. A wind turbine as claimed in claim 9 furthercomprising a controller, responsive to an output of the lidar, forcontrolling the pitch of the rotor blades.
 20. A wind turbine as claimedin claim 19 wherein the controller is adapted to feather the rotorblades when incoming wind gusts are detected.
 21. A wind turbine asclaimed in claim 19 wherein the controller is adapted to independentlyalter the pitch of each blade as it rotates.
 22. A wind turbine asclaimed in claim 21 wherein the controller is adapted to alter the pitchof each blade to maximise energy extraction.
 23. A wind turbine asclaimed in claim 21 wherein the controller is adapted to alter the pitchof each blade to provide load balancing.